SiBCN / SiO2 composite ceramic material, preparation method and application thereof

Abstract

The present application relates to the technical field of wave-transparent materials, and particularly relates to a SiBCN / SiO2 composite ceramic material, a preparation method and application thereof. The SiBCN / SiO2 composite ceramic material is prepared by the following steps: firstly, a precursor polyborosilazane is prepared by a polymer precursor conversion method; secondly, the precursor polyborosilazane is preliminarily crosslinked to obtain a heat-crosslinked precursor; thirdly, the heat-crosslinked precursor is mixed with SiO2 and then ball-milled; and finally, pyrolysis is performed to obtain the SiBCN / SiO2 composite ceramic material. The present application takes into account the technical defect of poor mechanical properties of the prior art SiBCN ceramic, and adds a functional filler SiO2 and performs pyrolysis to prepare the SiBCN / SiO2 composite ceramic material, so as to improve the mechanical properties, wave-transparent properties and heat resistance of the SiBCN ceramic.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wave-transparent materials, in particular to a SiBCN / SiO2 composite ceramic material and a preparation method and application thereof. BACKGROUND

[0002] A radar antenna is a precision device for detecting targets by using electromagnetic waves, and its internal structure is precise and complex, but its bearing capacity is poor. The wave-transparent material can be used as a protective material to protect the internal structure of the radar from being damaged. At present, the wave-transparent material is widely used in aircraft and ground radar equipment. With the continuous improvement of the speed of the aircraft, the wave-transparent material at the front end needs to withstand more and more extreme environmental tests. The radar antenna cover at the nose cone position has increasingly stringent requirements for material performance. The wave-transparent material needs to accurately transmit electromagnetic waves under the environment of high load, high heat flow and large torque. Therefore, it is of great significance to develop a wave-transparent material with high temperature resistance, oxidation resistance, low dielectric, and high strength.

[0003] It can be seen that the wave-transparent material not only requires the material to have high mechanical properties, impact resistance, environmental resistance, etc., but also requires the material to be able to transmit electromagnetic waves and almost not change the properties of the electromagnetic waves. The material needs to have low dielectric constant, high temperature resistance, etc.

[0004] SiBCN ceramics prepared by polymer precursor conversion method are widely used in the field of electromagnetic functional materials due to their adjustable dielectric properties and high temperature resistance. However, their mechanical properties are poor, which limits their development as structural bearing members. In addition, during the pyrolysis process of SiBCN ceramics prepared by polymer precursor conversion method, the overflow of small molecule gas will lead to a large number of pores in the ceramics, which directly affects the mechanical properties of the ceramics. Therefore, a ceramic material with excellent mechanical properties is needed to overcome the technical defects of existing SiBCN ceramics. SUMMARY

[0005] In view of the technical defects of the existing SiBCN ceramics, the SiBCN / SiO2 composite ceramic material is prepared by adding functional filler SiO2 to improve the mechanical properties, wave-transparent properties and heat resistance of SiBCN ceramics.

[0006] To achieve the above purpose, the technical scheme of the present application is as follows:

[0007] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0008] The precursor polyborosilazane is prepared by condensation method;

[0009] The precursor polysilazaborane is preliminarily cross-linked to obtain a thermal cross-linking precursor, and the purpose of the preliminary cross-linking is: a, to make the sensitive active groups react chemically, and the precursor is changed from a liquid state to a solid state, at which time the precursor becomes insensitive to air; b, the precursor will release gas during pyrolysis, which helps to make the sintering process after pressing easier; c, to help improve the ceramic yield; the thermal cross-linking precursor is mixed with SiO2 and then ball milled, and the precursor is an organic material which must be pyrolyzed to change from an organic material to an inorganic material to obtain a SiBCN / SiO2 composite ceramic material.

[0010] Preferably, the method further comprises the step of: before pyrolysis, pressing the ball-milled mixture of the thermal cross-linking precursor and SiO2 into a ceramic green body, which is convenient for preparation after pressing.

[0011] Preferably, the precursor polysilazaborane is prepared according to the following steps:

[0012] Trichlorosilane and boron trichloride are mixed, hexamethyldisilazane is added under ice bath, then stirring at room temperature for 8-10 h, and then the reaction system is heated to 230-250℃ and kept for 2-2.5 h to obtain the precursor polysilazaborane; too high temperature and too long reaction time will cause the precursor to cross-link prematurely, and too low temperature or too short time will cause the molecular weight of the precursor polysilazaborane to be too small, which are all not conducive to the preparation of the ceramic precursor, but the extension of the time cannot make up for the influence of too low temperature, and the reaction will not further proceed in the positive direction at low temperature.

[0013] Preferably, the molar ratio of trichlorosilane, boron trichloride and hexamethyldisilazane is 0.1:0.1:0.36-0.42.

[0014] Preferably, the preliminary cross-linking conditions are: cross-linking at 380-400℃ for 3-4 h in an inert atmosphere; the preliminary cross-linking conditions are common experience values obtained through experiments, and too low temperature and too short time will cause incomplete thermal cross-linking, and too high temperature will cause excessive cross-linking and possible organic-to-inorganic conversion, which will cause pressing difficulty, and the general time is 3-4 h.

[0015] Preferably, SiO2 accounts for 5-30% of the total amount of the thermal cross-linking precursor and SiO2, and this conclusion is obtained through experiments in the present application Figure 2 , Figure 2 The product with excellent mechanical properties corresponds to SiO2 accounting for 5-30% of the total amount of the thermal cross-linking precursor and SiO2.

[0016] Preferably, SiO2 accounts for 30% of the total amount of the thermal cross-linking precursor and SiO2, and this conclusion is obtained through experiments in the present application Figure 2The mechanical properties are the best at this time; the crosslinked precursor is pyrolyzed to cause mass loss, and the composite material is generated after pyrolysis, so the specific content of SiO2 cannot be tested, and thus the mass fraction of SiO2 in the total amount of the heat-crosslinked precursor and SiO2 is calculated.

[0017] Preferably, the pyrolysis reaction is carried out at 1350-1400 DEG C for 3-4 h in an inert atmosphere; the mechanical properties of pyrolysis at 1000-1400 DEG C are also tested, and the results show that the SiO2 softens only after 1400 DEG C, and the mechanical properties are all poor before 1300 DEG C, so the SiO2 does not soften at a lower temperature, the bonding and caulking ability of the material are reduced, and thus the mechanical properties are poor. Due to the limited experimental conditions, higher pyrolysis temperature tests are not carried out, but the increase of temperature will lead to the increase of the dielectric constant of the material and the decrease of the wave-penetrating ability of the material; the length of time is an empirical value, and the influence of time is not clear, but the material may not be completely converted if the time is too short, and the influence should be small if the time is longer.

[0018] The application also protects the SiBCN / SiO2 composite ceramic material prepared by the above preparation method.

[0019] The application also protects the application of the above SiBCN / SiO2 composite ceramic material in the preparation of electromagnetic wave-penetrating materials, and the results show that the SiBCN / SiO2 composite ceramic material has excellent wave-penetrating performance, and is expected to be applied in the preparation of electromagnetic wave-penetrating materials.

[0020] Compared with the prior art, the application has the following beneficial effects:

[0021] 1. In the application, the SiBCN ceramic material with high-temperature wave-penetrating performance is prepared by using carbon-free chlorosilane; the ceramic precursor prepared by using carbon-free chlorosilane can effectively reduce the carbon content in the precursor, and further reduce the carbon content in the ceramic, and improve the wave-penetrating rate of the ceramic.

[0022] 2. Quartz (SiO2) has a low dielectric constant, low dielectric loss and low softening temperature, and is an optimal filler for improving the mechanical properties of the SiBCN ceramic; in the research, it is found that with the increase of the pyrolysis temperature, the SiO2 can fill the micropores and cracks of the SiBCN ceramic matrix when softening, and even bond them, so as to improve the mechanical properties; in addition, the disappearance of the micropores and cracks can hinder the air from entering the ceramic system, and can improve the heat resistance and oxidation resistance of the ceramic system, and the reduction of the pores and cracks can also hinder the moisture in the air from entering the ceramic, and effectively reduce the hygroscopicity of the composite ceramic.

[0023] 3、The application fills the material by the low melting point of the silicon dioxide, improves the mechanical properties and heat resistance of the material, the silicon dioxide softens, fills the gap of the ceramic, and the oxygen in the air is difficult to erode the inside of the ceramic, so the heat resistance is improved. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The volume density and apparent porosity diagram of the SiBCN / SiO2 composite ceramic materials of Examples 2-4 and Comparative Examples 3-4, and the SiBCN ceramic material of Comparative Example 1.

[0025] Figure 2 The bending strength diagram of the SiBCN / SiO2 composite ceramic materials of Examples 1-4 and Comparative Examples 1-4.

[0026] Figure 3 The stress-strain curve diagram of the SiBCN / SiO2 composite ceramic materials of Comparative Examples 5-8 and Example 4.

[0027] Figure 4 The two-dimensional wave transmission performance curve diagram of the SiBCN ceramic material of Comparative Example 1 at room temperature-800 DEG C; wherein (a) is 25 DEG C, (b) is 100 DEG C, (c) is 200 DEG C, (d) is 300 DEG C, (e) is 400 DEG C, (f) is 500 DEG C, (g) is 600 DEG C, (h) is 700 DEG C, (i) is 800 DEG C.

[0028] Figure 5 The two-dimensional wave transmission performance curve diagram of the SiBCN / SiO2 ceramic material of Example 4 at room temperature-800 DEG C; wherein (a) is 25 DEG C, (b) is 100 DEG C, (c) is 200 DEG C, (d) is 300 DEG C, (e) is 400 DEG C, (f) is 500 DEG C, (g) is 600 DEG C, (h) is 700 DEG C, (i) is 800 DEG C.

[0029] Figure 6 The heat resistance diagram of the SiBCN / SiO2 ceramic materials of Examples 2-4, Comparative Example 1, Comparative Examples 3-4, wherein the left diagram is the thermal weight loss curve diagram of the SiBCN / SiO2 ceramic materials of Examples 2-4, Comparative Example 1, Comparative Examples 3-4 under argon atmosphere, and the right diagram is the thermal weight loss curve diagram of the SiBCN / SiO2 ceramic materials of Examples 2-4, Comparative Example 1, Comparative Examples 3-4 under air atmosphere.

[0030] Figure 7 The fracture scanning electron microscope diagram of the SiBCN / SiO2 ceramic materials of Examples 2-4, Comparative Example 1, Comparative Examples 3-4 pyrolyzed at 1400 DEG C; wherein (a) is SS-0, (b) is SS-10, (c) is SS-20, (d) is SS-30, (e) is SS-40, (f) is SS-50. Detailed Implementation

[0031] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0032] This invention aims to improve the mechanical properties of SiBCN ceramics while also enhancing their high-temperature microwave absorption properties. The introduction of SiO2 can significantly reduce porosity and increase the density of SiBCN / SiO2 ceramics. Furthermore, SiO2 softens at 1400℃, which helps to bind SiBCN ceramic particles together, further improving the mechanical properties of the SiBCN / SiO2 composite ceramic.

[0033] This invention also conducted a comparative study on the mechanical properties, heat resistance, and wave transmission properties of SiBCN / SiO2 composite ceramic materials. Samples were prepared with SiO2 comprising 0%, 2%, 5%, 10%, 20%, 30%, 40%, and 50% of the total mass of the thermally crosslinked precursor and SiO2, and samples were prepared at pyrolysis temperatures of 1000℃, 1100℃, 1200℃, 1300℃, and 1400℃. The results showed that, in terms of mechanical properties, when the mass fraction of SiO2 particles in the thermally crosslinked precursor and SiO2 was 30%, the flexural strength of the composite ceramic was 90.17 MPa, a significant improvement compared to the 3.89 MPa of the ceramic without added SiO2. In terms of wave transmission performance, the SiBCN / SiO2 composite ceramic material with a SiO2 content of 30% by mass after doping with nano-SiO2 particles has an average wave transmission rate of 86.76% in the X-band, and an average wave transmission rate of 85.03% at an ambient temperature of 800℃, a decrease of only 1.73%. Regarding heat resistance, the SiO2 / SiBCN ceramic material also exhibits excellent high-temperature stability in wave transmission performance. Thermogravimetric analysis shows that the SiBCN / SiO2 composite ceramic material with a SiO2 content of 30% by mass experiences only a 0.12% mass loss in an argon atmosphere at 1400℃, and a mass change of only 0.21% in an air atmosphere, representing a significant improvement compared to the SiBCN ceramic material without SiO2.

[0034] Example 1

[0035] The preparation method of SiBCN / SiO2 composite ceramic materials includes the following steps:

[0036] (1) trichlorosilane 0.1 mol, boron trichloride 1.0 mol / L, 100 mL were added into a reaction bottle, 0.42 mol of hexamethyldisilazane was added at low temperature 0℃ (i.e. ice bath), the whole reaction system was stirred at room temperature for 8 h, then the reaction system was gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain a precursor polysilazane;

[0037] (2) the precursor polysilazane of step (1) was transferred to a tube furnace, and heat crosslinking was carried out at 400℃ for 3 h, and then taken out after cooling to obtain a heat crosslinking precursor, SiO2 was mixed with the heat crosslinking precursor, at this time, SiO2 accounted for 5% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 was 5%, and ball milling was carried out for 1 h to fully mix and uniformly mix the two, and then a ceramic green body was pressed by a pressing mold, and then pyrolysis was carried out at 1400℃ for 4 h to obtain a SiBCN / SiO2 composite ceramic material, which was recorded as SS-5.

[0038] Example 2

[0039] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0040] (1) trichlorosilane 0.1 mol, boron trichloride 0.1 mol were added into a reaction bottle, 0.42 mol of hexamethyldisilazane was added at low temperature 0℃ (i.e. ice bath), the whole reaction system was stirred at room temperature for 8 h, then the reaction system was gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain a precursor polysilazane;

[0041] (2) the precursor polysilazane of step (1) was transferred to a tube furnace, and heat crosslinking was carried out at 400℃ for 3 h, and then taken out after cooling to obtain a heat crosslinking precursor, SiO2 was mixed with the heat crosslinking precursor, at this time, SiO2 accounted for 10% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 was 10%, and ball milling was carried out for 1 h to fully mix and uniformly mix the two, and then a ceramic green body was pressed by a pressing mold, and then pyrolysis was carried out at 1400℃ for 4 h to obtain a SiBCN / SiO2 composite ceramic material, which was recorded as SS-10.

[0042] Example 3

[0043] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0044] (1) trichlorosilane 0.1 mol, boron trichloride 0.1 mol are added to a reaction bottle, 0.42 mol of hexamethyldisilazane is added at low temperature 0℃ (i.e. ice bath), the whole reaction system is stirred at room temperature for 8 h, the reaction system is gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain a precursor polysilazane;

[0045] (2) the precursor polysilazane of step (1) is transferred to a tube furnace, and heat crosslinking is carried out at 400℃ for 3 h, and after cooling, the heat crosslinking precursor is taken out, SiO2 is mixed with the heat crosslinking precursor, at this time, SiO2 accounts for 20% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 is 20%, ball milling is carried out for 1 h, the two are fully mixed and uniform, a press mold is used to press into a ceramic green body, and then pyrolysis is carried out at 1400℃ for 4 h, to obtain a SiBCN / SiO2 composite ceramic material, which is recorded as SS-20.

[0046] Example 4

[0047] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0048] (1) trichlorosilane 0.1 mol, boron trichloride 0.1 mol are added to a reaction bottle, 0.42 mol of hexamethyldisilazane is added at low temperature 0℃ (i.e. ice bath), the whole reaction system is stirred at room temperature for 8 h, the reaction system is gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain a precursor polysilazane;

[0049] (2) the precursor polysilazane of step (1) is transferred to a tube furnace, and heat crosslinking is carried out at 400℃ for 3 h, and after cooling, the heat crosslinking precursor is taken out, SiO2 is mixed with the heat crosslinking precursor, at this time, SiO2 accounts for 20% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 is 20%, ball milling is carried out for 1 h, the two are fully mixed and uniform, a press mold is used to press into a ceramic green body, and then pyrolysis is carried out at 1400℃ for 4 h, to obtain a SiBCN / SiO2 composite ceramic material, which is recorded as SS-20.

[0050] Example 5

[0051] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0052] (1) trichlorosilane 0.1 mol, boron trichloride 0.1 mol are added to a reaction bottle, 0.4 mol of hexamethyldisilazane is added at low temperature 0℃ (i.e. ice bath), the whole reaction system is stirred at room temperature for 9 h, the reaction system is gradually heated to 230℃ at a heating rate of 50℃ / h, and kept for 2.5 h, to obtain a precursor polysilazane;

[0053] (2) the precursor polysilazane of step (1) is transferred to a tube furnace, and heat crosslinking is performed at 380℃ for 4 h, after cooling, the heat crosslinking precursor is taken out, SiO2 is mixed with the heat crosslinking precursor, at this time, SiO2 accounts for 30% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 is 30%, ball milling is performed for 1 h, the two are fully mixed and uniform, a press mold is used to press into a ceramic green body, and then pyrolysis is performed at 1350℃ for 4 h, to obtain a SiBCN / SiO2 composite ceramic material.

[0054] Example 6

[0055] A method for preparing a SiBCN / SiO2 composite ceramic material, comprising the following steps:

[0056] (1) trichlorosilane 0.1 mol, boron trichloride 0.1 mol are added to a reaction bottle, 0.36 mol of hexamethyldisilazane is added at low temperature 0℃ (i.e. ice bath), the whole reaction system is stirred at room temperature for 9 h, the reaction system is gradually heated to 230℃ at a heating rate of 50℃ / h, and kept for 2.5 h, to obtain a precursor polysilazane;

[0057] (2) the precursor polysilazane of step (1) is transferred to a tube furnace, and heat crosslinking is performed at 390℃ for 3.5 h, after cooling, the heat crosslinking precursor is taken out, SiO2 is mixed with the heat crosslinking precursor, at this time, SiO2 accounts for 30% of the total amount of the heat crosslinking precursor and SiO2, i.e. the mass fraction of SiO2 is 30%, ball milling is performed for 1 h, the two are fully mixed and uniform, a press mold is used to press into a ceramic green body, and then pyrolysis is performed at 1400℃ for 3.5 h, to obtain a SiBCN / SiO2 composite ceramic material.

[0058] Comparative Example 1

[0059] A method for preparing a SiBCN ceramic material, comprising the following steps:

[0060] Trichlorosilane 0.1 mol, boron trichloride 0.1 mol were added into a reaction bottle, 0.42 mol of hexamethyldisilazane was added at low temperature 0℃ (i.e. ice bath), the whole reaction system was stirred at room temperature for 8 h, then the reaction system was gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain the precursor polysilazane; the precursor was quickly transferred to a tube furnace, and preliminary crosslinking was performed at 400℃ for 4 h under argon atmosphere at a rate of 2℃ / min, and then cooled to room temperature; the powder was ground by a ball mill to pass through a 200 mesh screen, and the powder was pressed into a green body by a die mold, and then sintered at 1000℃ to obtain a SiBCN ceramic material, which is denoted as SS-0.

[0061] Comparative Example 2

[0062] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0063] (1) Trichlorosilane 0.1 mol, boron trichloride 0.1 mol were added into a reaction bottle, 0.42 mol of hexamethyldisilazane was added at low temperature 0℃ (i.e. ice bath), the whole reaction system was stirred at room temperature for 8 h, then the reaction system was gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain the precursor polysilazane;

[0064] (2) The precursor polysilazane of step (1) was transferred to a tube furnace, and preliminary crosslinking was performed at 400℃ for 3 h, and then taken out after cooling to obtain a heat-crosslinked precursor; SiO2 was mixed with the heat-crosslinked precursor, at this time, the SiO2 accounted for 2% of the total amount of the heat-crosslinked precursor and SiO2, i.e. the mass fraction of SiO2 was 2%, and the two were fully mixed and uniformly mixed by ball milling for 1 h, and then pressed into a ceramic green body by a die mold, and then pyrolyzed at 1400℃ for 4 h to obtain a SiBCN / SiO2 composite ceramic material, which is denoted as SS-2.

[0065] Comparative Example 3

[0066] The preparation method of the SiBCN / SiO2 composite ceramic material comprises the following steps:

[0067] (1) Trichlorosilane 0.1 mol, boron trichloride 0.1 mol were added into a reaction bottle, 0.42 mol of hexamethyldisilazane was added at low temperature 0℃ (i.e. ice bath), the whole reaction system was stirred at room temperature for 8 h, then the reaction system was gradually heated to 250℃ at a heating rate of 50℃ / h, and kept for 2 h, to obtain the precursor polysilazane;

[0068] (2) The precursor polysilazaborane of step (1) is transferred to a tube furnace, and heat crosslinking is performed at 400°C for 3h, and after cooling, the heat crosslinking precursor is taken out. SiO2 is mixed with the heat crosslinking precursor, at this time, the SiO2 accounts for 40% of the total amount of the heat crosslinking precursor and SiO2, that is, the mass fraction of SiO2 is 40%, and ball milling is performed for 1h to fully mix and uniformly mix the two, and a ceramic green body is pressed by a press mold, and then pyrolysis is performed at 1400°C for 4h to obtain a SiBCN / SiO2 composite ceramic material, which is denoted as SS-40.

[0069] Comparative Example 4

[0070] A method for preparing a SiBCN / SiO2 composite ceramic material includes the following steps:

[0071] (1) 0.1 mol of trichlorosilane and 0.1 mol of boron trichloride are added to a reaction bottle, 0.42 mol of hexamethyldisilazane is added at a low temperature of 0°C (i.e., ice bath), the entire reaction system is stirred at room temperature for 8h, and then the reaction system is gradually heated to 250°C at a heating rate of 50°C / h and maintained for 2h to obtain a precursor polysilazaborane;

[0072] (2) The precursor polysilazaborane of step (1) is transferred to a tube furnace, and heat crosslinking is performed at 400°C for 3h, and after cooling, the heat crosslinking precursor is taken out. SiO2 is mixed with the heat crosslinking precursor, at this time, the SiO2 accounts for 40% of the total amount of the heat crosslinking precursor and SiO2, that is, the mass fraction of SiO2 is 40%, and ball milling is performed for 1h to fully mix and uniformly mix the two, and a ceramic green body is pressed by a press mold, and then pyrolysis is performed at 1400°C for 4h to obtain a SiBCN / SiO2 composite ceramic material, which is denoted as SS-40.

[0073] Comparative Example 5

[0074] The preparation steps are the same as those of Example 4, except that the pyrolysis temperature in step (2) is replaced by 1000°C instead of 1400°C, which is denoted as SS-30-1000.

[0075] Comparative Example 6

[0076] The preparation steps are the same as those of Example 4, except that the pyrolysis temperature in step (2) is replaced by 1100°C instead of 1400°C, which is denoted as SS-30-1100.

[0077] Comparative Example 7

[0078] The preparation steps are the same as those of Example 4, except that the pyrolysis temperature in step (2) is replaced by 1200°C instead of 1400°C, which is denoted as SS-30-1200.

[0079] Comparative Example 8

[0080] The preparation steps of Examples 1-4 and Comparative Examples 2-4 are the same, except that the pyrolysis temperature in step (2) is changed from 1400°C to 1300°C, denoted as SS-30-1300.

[0081] The preparation steps of Examples 1-4 and Comparative Examples 2-4 are the same, except that the mass fraction of SiO2 in the total amount of the heat-crosslinking precursor and SiO2 is different. Comparative Example 1 of the present application is a SiBCN ceramic material without SiO2 composite, and in Comparative Examples 2, 3, and 4, SiO2 accounts for 2%, 40%, and 50% of the total amount of the heat-crosslinking precursor and SiO2, respectively; in Examples 1-4, SiO2 accounts for 5%, 10%, 20%, and 30% of the total amount of the heat-crosslinking precursor and SiO2, respectively. The following takes the samples of Examples 1-4 and Comparative Examples 1-4 as examples for comparative study, and the specific research methods and results are shown below:

[0082] The density is measured by the Archimedes drainage method, and the porosity is measured by a specific surface area analyzer. Figure 1 The results show that as the amount of SiO2 added increases, the porosity of the material gradually decreases, and the density gradually increases, indicating that SiO2 effectively fills the pores inside the ceramic.

[0083] The mechanical property test method is as follows: an electronic universal testing machine of Xinsansi (Shanghai) Enterprise Development Co., Ltd. is used, the model is CMT 5105, the power supply is 380V, the working power is 2KW, the displacement control rate is 0.5mm / min, the three-point bending method is used to test the bending strength of the ceramic long sheet, and the length of the ceramic long sheet is 5cm, the width is 1.2cm, and the thickness is 2.5mm.

[0084] Figure 2 The results show that when SiO2 accounts for 30% of the total amount of the heat-crosslinking precursor and SiO2, the composite ceramic has the best mechanical properties, and the bending strength at this time is 90.17MPa.

[0085] Figure 3 The stress-strain curves of the SiO2 / SiBCN ceramic materials of Example 4 and Comparative Examples 5-8 are shown in the figure. As can be seen from the figure, as the pyrolysis temperature increases from 1000°C to 1400°C, the bending strength of the SiO2 / SiBCN ceramic gradually increases, and when the pyrolysis temperature is 1400°C, the bending strength increases sharply to 95.23MPa, which is a relatively obvious increase compared with 50.93MPa at 1300°C. This is because the nano-silicon dioxide has softened at this time, filling and bonding the gaps between the SiBCN ceramic particles, thereby significantly improving the bending strength of the ceramic.

[0086] The detection method of the wave-transparent performance: the electromagnetic parameters of the ceramic sample are measured by the vector network analyzer produced by the Japanese Anristu company, the model is MS4644A, the method is waveguide method, the frequency band is X band (8.2-12.4 GHz), Ku band (12.4-18 GHz). The sample to be measured is polished into a standard size of 22.86 mm x 10.15 mm and 15.85 mm x 7.95 mm, before measurement, the instrument is calibrated by using a polytetrafluoroethylene standard sample, then the real part and the imaginary part of the relative dielectric constant of the ceramic sample and other parameters are measured, the wave-transparent performance is calculated according to the dielectric constant, and the specific calculation method is:

[0087] Generally, for the wave-transparent material, the relationship among the power transmission coefficient │T 2 │ (i.e. the wave-transparent rate), the power reflection coefficient │R 2 │ and the electromagnetic wave loss A (mainly the heat loss) in the ceramic is:

[0088] │T 2 │+│R 2 │+A=1

[0089] Suppose that the medium is a non-loss medium, and taking the horizontally polarized electromagnetic wave as an example, the electromagnetic wave energy loss A, the power reflection coefficient r and the wave-transparent rate │T 2 can be expressed as:

[0090] Aπ=(2πd / λ)[(εtgδ) / (δ-sin 2 θ) 1 / 2 ] (3-1)

[0091] r=(1-n ab ) / (1+n ab ) (3-2)

[0092]

[0093] Wherein the refractive index n ab =εcosθ / (ε-sin 2 θ) 1 / 2

[0094] is the phase shift caused by the incident wave through the medium plate with a thickness of d.

[0095] In the formula: d--plate thickness; λ--wavelength; θ--incident angle; ε--dielectric constant; tgδ--dielectric loss angle tangent;

[0096] Through Figure 4 and Figure 5The data of SiBCN ceramic material of Comparative Example 1 and SiO2 / SiBCN ceramic material of Example 4 at different temperatures were obtained and calculated according to the above formula, and the results are shown in Table 1.

[0097] Table 1: Comparison of wave transmission performance

[0098]

[0099] Table 1 shows that the wave transmission performance of SiO2 / SiBCN ceramic material and SiBCN ceramic material gradually decreases as the test temperature increases. Although the temperature reduces the wave transmission performance of SiO2 / SiBCN ceramic material, the SiO2 / SiBCN ceramic material still maintains a high wave transmission rate even at 800℃.

[0100] Figure 6 The results show that the heat resistance of SiO2 / SiBCN ceramic material is excellent. Under the conditions of 1400℃, argon and air atmosphere, the mass of SiO2 / SiBCN ceramic material does not change significantly, while the mass of SiBCN ceramic material changes significantly, indicating that the doping of SiO2 effectively improves the heat resistance of SiBCN ceramic material.

[0101] Figure 7 (a)-(f) show the fracture surface scanning electron microscope images of SiBCN / SiO2 ceramic materials of Examples 2-4, Comparative Example 1, Comparative Examples 3-4 after bending and breaking at 1400℃. Figure 7 (a) shows a pure SiBCN ceramic material, which has a large number of cracks and pores on the fracture surface. This is because a part of gas escapes during the pyrolysis of polyborosilazane precursor to ceramic, resulting in volume shrinkage, which causes cracks and pore defects in the ceramic matrix. These defects not only reduce the density of the ceramic, but also damage its continuity, reducing the effective bearing area of the matrix, and further reducing the bending fracture performance of the ceramic. The pores between ceramic particles will cause a large number of ceramic particles to be unable to participate in bearing, and these cracks and pores will also become the fracture source of ceramic bending failure. These defects will cause stress concentration and serious damage under stress, which further reduces the bending strength of the ceramic.

[0102] From Figure 7 (b)-(f) show that the introduction of nano-silica significantly reduces the open porosity of SiBCN / SiO2 ceramic matrix, increases the density of the ceramic matrix, and further reduces the cracks and pores of SiBCN / SiO2 ceramic, improving the continuity of the ceramic matrix. Moreover, nano-silica will soften at a pyrolysis temperature of 1400℃, and the silica in the pores will not only fill but also have a cohesive effect, further improving the bending performance of SiBCN / SiO2 ceramic.

[0103] When SiO2 accounts for 30% of the total amount of the heat-crosslinking precursor and SiO2, the bending strength of SiBCN / SiO2 is the largest, and the bending strength is 31.48 MPa, which is 1.5 times of that of pure SiBCN. Figure 7 (d) It can be seen that the pores and cracks are obviously reduced compared with pure SiBCN, and some characteristics of brittle fracture can be seen here, such as mirror zone, haze zone, etc. With the increase of the content of nano-silica, the bending strength of SiBCN / SiO2 is reduced, and the bending strength is 31.48 MPa, which is 1.5 times of that of pure SiBCN. Figure 7 (e) and Figure 7 (f) The typical brittle material fracture morphology can also be seen, including obvious mirror zone, feather zone, etc. It can also be seen that when the mass fraction of nano-silica is increased to 50%, the density and continuity of SiBCN / SiO2 ceramic are further improved, and only a few cracks and pores can be seen, but the bending strength of the ceramic is sharply reduced, only 9.01 MPa, because the existence of appropriate cracks and pores in the ceramic is beneficial to the stress release of the sample under stress, but when the cracks and pores are reduced to a certain extent, the stress release will be blocked, and the fracture toughness of the material will be sharply reduced, so the SS-50-1400 sample shows a sharp decrease in fracture strength.

[0104] In addition, during the process of converting the precursor into ceramic by pyrolysis, the thermal expansion coefficients of SiBCN ceramic and SiO2 nanoparticles are different, which will cause thermal mismatch, and especially when the density and continuity of SiBCN / SiO2 composite ceramic continue to increase, thermal stress will be generated in the ceramic matrix to cause the existence of some micro-cracks. In summary, the introduction of appropriate content of nano-silica particles is beneficial to improve the density and continuity of SiBCN / SiO2 ceramic, reduce the porosity, and thus improve the bending strength of the material.

[0105] In addition, during the process of converting the precursor into ceramic by pyrolysis, the thermal expansion coefficients of SiBCN ceramic and SiO2 nanoparticles are different, which will cause thermal mismatch, and especially when the density and continuity of SiBCN / SiO2 composite ceramic continue to increase, thermal stress will be generated in the ceramic matrix to cause the existence of some micro-cracks. In summary, the introduction of appropriate content of nano-silica particles is beneficial to improve the density and continuity of SiBCN / SiO2 ceramic, reduce the porosity, and thus improve the bending strength of the material.

[0106] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application belong to the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these modifications and variations.

Claims

1. A method for preparing SiBCN / SiO2 composite ceramic material, characterized in that, The method comprises the following steps: The precursor polyborosilazane is prepared by condensation polymerization, and the precursor polyborosilazane is prepared by the following steps: mixing trichlorosilane and boron trichloride, adding hexamethyldisilazane under ice bath, stirring at room temperature for 8-10 h, and then performing polymerization reaction on the reaction system to obtain the precursor polyborosilazane; After the precursor polyborosilazane is preliminarily crosslinked, a thermal crosslinking precursor is obtained, the thermal crosslinking precursor is mixed with SiO2 and then ball milled, and then pyrolysis is performed on the mixture under an inert atmosphere at 1350-1400 ℃ to obtain the SiBCN / SiO2 composite ceramic material. The SiO2 accounts for 5-30% of the total mass of the thermal crosslinking precursor and SiO2.

2. The method for preparing the SiBCN / SiO2 composite ceramic material according to claim 1, characterized in that, The method further comprises the following step: before pyrolysis, the ball-milled mixture of the thermal crosslinking precursor and SiO2 is pressed into a ceramic green body.

3. The method for preparing the SiBCN / SiO2 composite ceramic material according to claim 1, characterized in that, The molar ratio of the trichlorosilane, boron trichloride and hexamethyldisilazane is 0.1:0.1:0.36-0.42, and the polymerization reaction is performed at 230-250 ℃ for 2-2.5 h.

4. The method for preparing the SiBCN / SiO2 composite ceramic material according to claim 1, characterized in that, The preliminary crosslinking is performed at 380-400 ℃ for 3-4 h under an inert atmosphere.

5. The method for preparing the SiBCN / SiO2 composite ceramic material according to claim 1, characterized in that, The SiO2 accounts for 30% of the total mass of the thermal crosslinking precursor and SiO2.

6. The method for preparing the SiBCN / SiO2 composite ceramic material according to claim 1, characterized in that, The pyrolysis is performed for 3-4 h.

7. A SiBCN / SiO2 composite ceramic material prepared by the method of any one of claims 1-6.

8. Use of the SiBCN / SiO2 composite ceramic material of claim 7 in the preparation of electromagnetic wave transparent materials.

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